Carbon nanotube structure

ABSTRACT

The present disclosure relates to a carbon nanotube structure. The carbon nanotube structure includes a carbon nanotube array, a carbon nanotube layer located on the carbon nanotube array, and a carbon nanotube cluster between the carbon nanotube array and the carbon nanotube layer. The carbon nanotube array includes a number of first carbon nanotubes that are parallel with each other. The carbon nanotube layer includes a number of second carbon nanotubes. The carbon nanotube cluster includes a plurality of third carbon nanotubes that are entangled around both the plurality of first carbon nanotubes and the plurality of second carbon nanotubes. The carbon nanotube array is fixed on the carbon nanotube layer by the plurality of third carbon nanotubes so that the entire carbon nanotube structure is free-standing.

This application is a continuation application of U.S. patentapplication Ser. No. 13/711469, filed on Dec. 11, 2012, entitled “CARBONNANOTUBE STRUCTURE,” which claims all benefits accruing under 35 U.S.C.§ 119 from China Patent Applications: Application No. 201210135962.9,filed on May 4, 2012 in the China Intellectual Property Office,disclosures of which are incorporated herein by references.

BACKGROUND 1. Technical Field

The present disclosure relates to carbon nanotube structures, methodsfor making the same and field emission devices using the same.

2. Description of Related Art

Carbon nanotubes produced by means of arc discharge between graphiterods were first discovered and reported in an article by Sumio Iijima,entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354,Nov. 7, 1991, pp.56-58). Carbon nanotubes also feature extremely highelectrical conductivity, very small diameters (much less than 100nanometers), large aspect ratios, and a tip-surface area near thetheoretical limit. These features tend to make carbon nanotubes idealcandidates for electron emitter in field emission device.

In US20060192475A1 published on Aug. 31, 2006, Li et al. discloses acarbon nanotube emitter and its fabrication method. The carbon nanotubeemitter includes a plurality of first carbon nanotubes arranged on asubstrate and in parallel with the substrate, and a plurality of thesecond carbon nanotubes arranged on a surface of the first carbonnanotubes. The method for making the carbon nanotube emitter includes:growing a plurality of first carbon nanotubes on a first substratehaving a catalyst material layer arranged thereon; separating the firstcarbon nanotubes from the first substrate and immersing the firstseparated carbon nanotubes in a dispersion solution; coating a secondsubstrate with the dispersion solution and baking the second coatedsubstrate at a predetermined temperature to fix the first carbonnanotubes on the second substrate and in parallel with the secondsubstrate; and growing a plurality of second carbon nanotubes from aplurality of nano catalyst particles on the surface of the first carbonnanotubes.

However, the method for making the carbon nanotube emitter iscomplicated and the combination force between the first carbon nanotubesand the second carbon nanotubes are week. Thus, the carbon nanotubes ofthe carbon nanotube emitter are easy to be pulled out when it is used infield emission device.

What is needed, therefore, is to provide a carbon nanotube structure inwhich the carbon nanotubes are firmly fixed and not easy to be pulledout, and a simple method for making the same.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flowchart of one embodiment of a method for making a carbonnanotube structure.

FIG. 2 is a Scanning Electron Microscope (SEM) image of a drawn carbonnanotube film.

FIG. 3 is a schematic structural view of a carbon nanotube segment ofthe drawn carbon nanotube film of FIG. 2.

FIG. 4 is an SEM image of cross-stacked drawn carbon nanotube films.

FIG. 5 is an SEM image of an untwisted carbon nanotube wire.

FIG. 6 is an SEM image of a twisted carbon nanotube wire.

FIG. 7 is an SEM image of a pressed carbon nanotube film.

FIG. 8 is an SEM image of a flocculated carbon nanotube film.

FIG. 9 is a schematic view of one embodiment of a carbon nanotubestructure fabricated in the method of FIG. 1.

FIG. 10 is an SEM image of one embodiment of a carbon nanotube structurefabricated in the method of FIG. 1.

FIG. 11 is an SEM image of side view of one embodiment of the carbonnanotube structure of FIG. 10.

FIG. 12 is an SEM image of top view of one embodiment of the carbonnanotube structure of FIG. 10.

FIG. 13 is an SEM image of top side view of one embodiment of the carbonnanotube structure of FIG. 10.

FIG. 14 is an SEM image of bottom side view of one embodiment of thecarbon nanotube structure of FIG. 10.

FIG. 15 is an SEM image of bottom view of one embodiment of the carbonnanotube structure of FIG. 10.

FIG. 16 is a flowchart of one embodiment of a method for making a carbonnanotube structure.

FIG. 17 is a schematic view of one embodiment of a carbon nanotubestructure fabricated in the method of FIG. 16.

FIG. 18 is an SEM image of one embodiment of a suspended part of acarbon nanotube layer of the carbon nanotube structure fabricated in themethod of FIG. 16.

FIG. 19 is a flowchart of one embodiment of a method for making a carbonnanotube structure.

FIG. 20 is a flowchart of one embodiment of a method for making a carbonnanotube structure.

FIG. 21 is a schematic view of one embodiment of a field emission deviceusing the carbon nanotube structure of FIG. 9.

FIG. 22 is a photo of one embodiment of a field emission device usingthe carbon nanotube structure of FIG. 9.

FIGS. 23-26 are testing results of the field emission device of FIG. 22.

FIG. 27 is a schematic view of one embodiment of a field emission deviceusing the carbon nanotube structure of FIG. 9.

FIG. 28 is a schematic view of one embodiment of a field emission deviceusing the carbon nanotube structure of FIG. 9.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail,various embodiments of the present carbon nanotube structures andmethods for making the same.

Referring to FIG. 1, a method for making a carbon nanotube structure 100of one embodiment includes the following steps:

step (S11), providing a substrate 101 having an growing surface 105;

step (S12), placing a carbon nanotube layer 102 on the growing surface105 of the substrate 101, wherein part of the growing surface 105 isexposed through the carbon nanotube layer 102;

step (S13), depositing a plurality of first catalysts 104 on surface ofthe carbon nanotube layer 102 and depositing a plurality of secondcatalysts 106 on the growing surface 105; and

step (S14), growing a carbon nanotube array 110 on the growing surface105 and growing a carbon nanotube cluster 108 on the surface of thecarbon nanotube layer 102.

In step (S11), the substrate 101 has a growing surface 105 that is aclean and smooth surface. The growing surface 105 can be flat or curved.The growing surface 105 can be mechanically polished orelectrochemically polished. A smoothness of the growing surface 105 canbe less than 300 nanometers for facilitating a uniform formation of acatalyst layer directly on the substrate 101. The substrate 101 can be asilicon substrate, a silicon dioxide substrate, a quartz substrate, asapphire substrate, or a ceramic substrate. The size, thickness, andshape of the substrate 101 can be selected according to need. In oneembodiment, the substrate 101 is a silicon wafer with a size of 4-inch.

In step (S12), the carbon nanotube layer 102 is placed on and in contactwith the growing surface 105 of the substrate 101. The carbon nanotubelayer 102 is an integrated macrostructure in layer shape. The carbonnanotube layer 102 is a free-standing structure. The term “free-standingstructure” includes, but is not limited to, the fact that the carbonnanotube layer 102 can sustain the weight of itself when it is hoistedby a portion thereof without any significant damage to its structuralintegrity.

Thus, the carbon nanotube layer 102 can be suspended by two spacedsupports. The free-standing carbon nanotube layer 102 can be laid on thegrowing surface 105 directly and easily.

The carbon nanotube layer 102 includes a plurality of carbon nanotubes.The carbon nanotubes in the carbon nanotube layer 102 can besingle-walled, double-walled, or multi-walled carbon nanotubes. Thelength and diameter of the carbon nanotubes can be selected according toneed. The thickness of the carbon nanotube layer 102 can be in a rangefrom about 1 nanometer to about 100 micrometers. For example, thethickness of the carbon nanotube layer 102 can be about 10 nanometers,100 nanometers, 200 nanometers, 1 micrometer, 10 micrometers, or 50micrometers. The carbon nanotube layer 102 forms a patterned structure,therefore, part of the growing surface 105 can be exposed from thepatterned carbon nanotube layer 102 after the carbon nanotube layer 102is placed on the growing surface 105. The carbon nanotube layer 102 canbe a substantially pure structure of carbon nanotubes, with fewimpurities and chemical functional groups. The heat capacity per unitarea of the carbon nanotube layer 102 can be less than 2×10⁻⁴ J/m²*K. Inone embodiment, the heat capacity per unit area of the carbon nanotubelayer 102 is less than or equal to 1.7×10⁻⁶ J/m²*K.

The patterned carbon nanotube layer 102 defines a plurality ofapertures. The apertures can be dispersed uniformly. The apertureextends throughout the carbon nanotube layer 102 along the thicknessdirection thereof. The aperture can be a hole defined by severaladjacent carbon nanotubes, or a gap defined by two substantiallyparallel carbon nanotubes and extending along axial direction of thecarbon nanotubes. The hole shaped aperture and the gap shaped aperturecan exist in the patterned carbon nanotube layer 102 at the same time.Hereafter, the size of the aperture is the diameter of the hole or widthof the gap. The sizes of the apertures can be different. The averagesize of the apertures can be in a range from about 2 nanometers to about100 micrometers. For example, the sizes of the apertures can be about 10nanometers, 50 nanometers, 100 nanometers, 500 nanometers, 1 micrometer,5 micrometers, 10 micrometers, or 50 micrometers. When the size of theapertures is less than 100 micrometers, the carbon nanotube array 110grown in following step can lift the carbon nanotube layer 102 up awayfrom the growing surface 105. When the size of the apertures is toolarge, the carbon nanotube array 110 grown in following step will getthrough the apertures and the carbon nanotube layer 102 cannot be liftedup. In one embodiment, the sizes of the apertures are in a range fromabout 50 nanometers to about 100 nanometers. In order to deposit enoughsecond catalyst 106 on the growing surface 105, to grown the carbonnanotube array 110 in following step, the duty ratio of the carbonnanotube layer 102 can be in a range from about 95:5 to about 5:95. Forexample, the duty ratio of the carbon nanotube layer 102 can be about9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8 or 1:9. In one embodiment, theduty ratio of the carbon nanotube layer 102 is in a range from about 1:4to about 4:1. The duty ratio of the carbon nanotube layer 102 is an arearatio between the sheltered portions of the growing surface 105 and theexposed portions of the growing surface 105.

The carbon nanotubes of the carbon nanotube layer 102 can be orderlyarranged to form an ordered carbon nanotube structure or disorderlyarranged to form a disordered carbon nanotube structure. The term‘disordered carbon nanotube structure’ includes, but is not limited to,a structure wherein the carbon nanotubes are arranged along manydifferent directions, and the aligning directions of the carbonnanotubes are random. The number of the carbon nanotubes arranged alongeach different direction can be almost the same (e.g. uniformlydisordered). The disordered carbon nanotube structure can be isotropic.The carbon nanotubes in the disordered carbon nanotube structure can beentangled with each other. The term ‘ordered carbon nanotube structure’includes, but is not limited to, a structure wherein the carbonnanotubes are arranged in a consistently systematic manner, e.g., thecarbon nanotubes are arranged approximately along a same directionand/or have two or more sections within each of which the carbonnanotubes are arranged approximately along a same direction (differentsections can have different directions).

The carbon nanotube layer 102 can include at least one carbon nanotubefilm, at least one carbon nanotube wire, or combination thereof. In oneembodiment, the carbon nanotube layer 102 can include a single carbonnanotube film or two or more carbon nanotube films stacked together.Thus, the thickness of the carbon nanotube layer 102 can be controlledby the number of the stacked carbon nanotube films. The number of thestacked carbon nanotube films can be in a range from about 2 to about100. For example, the number of the stacked carbon nanotube films can be10, 30, or 50. In one embodiment, the carbon nanotube layer 102 caninclude a layer of parallel and spaced carbon nanotube wires. Also, thecarbon nanotube layer 102 can include a plurality of carbon nanotubewires crossed or weaved together to form a carbon nanotube net. It isunderstood that any carbon nanotube structure described can be used withall embodiments.

In one embodiment, the carbon nanotube layer 102 includes at least onedrawn carbon nanotube film. A drawn carbon nanotube film can be drawnfrom a carbon nanotube array that is able to have a film drawntherefrom. The drawn carbon nanotube film includes a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween. The drawn carbon nanotube film is afree-standing film. Referring to FIGS. 2 to 3, each drawn carbonnanotube film includes a plurality of successively oriented carbonnanotube segments 143 joined end-to-end by van der Waals attractiveforce therebetween. Each carbon nanotube segment 143 includes aplurality of carbon nanotubes 145 parallel to each other, and combinedby van der Waals attractive force therebetween. As can be seen in FIG.2, some variations can occur in the drawn carbon nanotube film. Thecarbon nanotubes 145 in the drawn carbon nanotube film are orientedalong a preferred orientation. The drawn carbon nanotube film can betreated with an organic solvent to increase the mechanical strength andtoughness and reduce the coefficient of friction of the drawn carbonnanotube film. A thickness of the drawn carbon nanotube film can rangefrom about 0.5 nanometers to about 100 micrometers. The drawn carbonnanotube film can be attached to the growing surface 105 directly.

The carbon nanotube layer 102 can include at least two stacked drawncarbon nanotube films. In other embodiments, the carbon nanotube layer102 can include two or more coplanar carbon nanotube films, and caninclude layers of coplanar carbon nanotube films. Additionally, when thecarbon nanotubes in the carbon nanotube film are aligned along onepreferred orientation (e.g., the drawn carbon nanotube film), an anglecan exist between the orientation of carbon nanotubes in adjacent films,whether stacked or adjacent. Adjacent carbon nanotube films can becombined by only the van der Waals attractive force therebetween. Anangle between the aligned directions of the carbon nanotubes in twoadjacent carbon nanotube films can range from about 0 degrees to about90 degrees. When the angle between the aligned directions of the carbonnanotubes in adjacent stacked drawn carbon nanotube films is larger than0 degrees, a plurality of micropores is defined by the carbon nanotubelayer 102. Referring to FIG. 4, the carbon nanotube layer 102 is shownwith the aligned directions of the carbon nanotubes between adjacentstacked drawn carbon nanotube films at 90 degrees. Stacking the carbonnanotube films will also add to the structural integrity of the carbonnanotube layer 102.

The carbon nanotube wire can be untwisted or twisted. Treating the drawncarbon nanotube film with a volatile organic solvent can form theuntwisted carbon nanotube wire. Specifically, the organic solvent isapplied to soak the entire surface of the drawn carbon nanotube film.During the soaking, adjacent parallel carbon nanotubes in the drawncarbon nanotube film will bundle together, due to the surface tension ofthe organic solvent as it volatilizes, and thus, the drawn carbonnanotube film will be shrunk into an untwisted carbon nanotube wire.Referring to FIG. 5, the untwisted carbon nanotube wire includes aplurality of carbon nanotubes substantially oriented along a samedirection (i.e., a direction along the length of the untwisted carbonnanotube wire). The carbon nanotubes are substantially parallel to theaxis of the untwisted carbon nanotube wire. More specifically, theuntwisted carbon nanotube wire includes a plurality of successive carbonnanotube segments joined end to end by van der Waals attractive forcetherebetween. Each carbon nanotube segment includes a plurality ofcarbon nanotubes substantially parallel to each other, and combined byvan der Waals attractive force therebetween. The carbon nanotubesegments can vary in width, thickness, uniformity, and shape. The lengthof the untwisted carbon nanotube wire can be arbitrarily set as desired.A diameter of the untwisted carbon nanotube wire ranges from about 0.5nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawncarbon nanotube film using a mechanical force to turn the two ends ofthe drawn carbon nanotube film in opposite directions. Referring to FIG.6, the twisted carbon nanotube wire includes a plurality of carbonnanotubes helically oriented around an axial direction of the twistedcarbon nanotube wire. More specifically, the twisted carbon nanotubewire includes a plurality of successive carbon nanotube segments joinedend to end by van der Waals attractive force therebetween. Each carbonnanotube segment includes a plurality of carbon nanotubes parallel toeach other, and combined by van der Waals attractive force therebetween.The length of the carbon nanotube wire can be set as desired. A diameterof the twisted carbon nanotube wire can be from about 0.5 nanometers toabout 100 micrometers. Further, the twisted carbon nanotube wire can betreated with a volatile organic solvent after being twisted to bundlethe adjacent paralleled carbon nanotubes together. The specific surfacearea of the twisted carbon nanotube wire will decrease, while thedensity and strength of the twisted carbon nanotube wire will increase.

In another embodiment, the carbon nanotube layer 102 can include apressed carbon nanotube film. Referring to FIG. 7, the pressed carbonnanotube film can be a free-standing carbon nanotube film. The carbonnanotubes in the pressed carbon nanotube film are arranged along a samedirection or arranged along different directions. The carbon nanotubesin the pressed carbon nanotube film can rest upon each other. Adjacentcarbon nanotubes are attracted to each other and combined by van derWaals attractive force. An angle between a primary alignment directionof the carbon nanotubes and a surface of the pressed carbon nanotubefilm is about 0 degrees to approximately 15 degrees. The greater thepressure applied, the smaller the angle formed. If the carbon nanotubesin the pressed carbon nanotube film are arranged along differentdirections, the carbon nanotube layer 102 can be isotropic.

In another embodiment, the carbon nanotube layer 102 includes aflocculated carbon nanotube film. Referring to FIG. 8, the flocculatedcarbon nanotube film can include a plurality of long, curved, disorderedcarbon nanotubes entangled with each other. Furthermore, the flocculatedcarbon nanotube film can be isotropic. The carbon nanotubes can besubstantially uniformly dispersed in the carbon nanotube film. Adjacentcarbon nanotubes are acted upon by van der Waals attractive force toform an entangled structure with micropores defined therein. Sizes ofthe micropores can be less than 10 micrometers. The porous nature of theflocculated carbon nanotube film will increase the specific surface areaof the carbon nanotube layer 102. Further, due to the carbon nanotubesin the carbon nanotube layer 102 being entangled with each other, thecarbon nanotube layer 102 employing the flocculated carbon nanotube filmhas excellent durability, and can be fashioned into desired shapes witha low risk to the integrity of the carbon nanotube layer 102. Theflocculated carbon nanotube film, in some embodiments, is free-standingdue to the carbon nanotubes being entangled and adhered together by vander Waals attractive force therebetween.

In step (S13), the catalyst can be deposited by a method of electronbeam evaporation, magnetron sputtering, plasma deposition,electro-deposition and thermal deposition. Because the carbon nanotubelayer 102 has a plurality of apertures, part of the catalyst isdeposited on the carbon nanotube layer 102, to form the first catalyst104, and the other part of the catalyst is deposited on growing surface105 through the apertures to form the second catalyst 106.

The material of the catalyst is a transition metal. Examples oftransitional metals are iron (Fe), cobalt (Co), nickel (Ni), platinum(Pt), palladium (Pd), or mixtures or alloys of the metals. Thedeposition rate of the catalyst can be can be less than 0.5 nm/s. Thethickness of the catalyst can be in a range from about 1 nanometer toabout 30 nanometers. In one embodiment, the catalyst is layer of ironwith a thickness of 5 nanometers to about 10 nanometers. Furthermore, astep of annealing can be performed in vacuum at a temperature in a rangefrom about 700° C. to about 900° C. for a time in a range from about 30minutes to about 90 minutes, thereby transforming the catalyst layerinto catalyst particles.

In another embodiment, the first catalyst 104 and the second catalyst106 can be a patterned catalyst layer. That is, the first catalyst 104is only deposited on part of the carbon nanotube layer 102, and thesecond catalyst 106 is only deposited on part of the growing surface105. For example, a patterned mask can be used to shelter part of thecarbon nanotube layer 102 and the growing surface 105 during the processof depositing catalyst, thereby obtaining a patterned first catalyst 104and a patterned second catalyst 106. Thus, a patterned carbon nanotubearray 110 and a patterned carbon nanotube cluster 108 can be achieved.

In step (S14), the carbon nanotube array 110 and the carbon nanotubecluster 108 are grown by the method of chemical vapor deposition. In oneembodiment, the step (14) includes the following substeps:

step (S141), placing the substrate 101 with the carbon nanotube layer102 thereon into a reacting room;

step (S142), introducing a carbon source gas and a protecting gas intothe reacting room; and

step (S143), heating the substrate 101 having the carbon nanotube layer102 thereon to a temperature in a range from about 300° C. to about1200° C.

In step (S141), the reacting room is a quartz tube in a quartz tubefurnace. The reacting room is further evacuated to form a vacuum beforestep (S142).

In step (S142), the protecting gas is introduced into the reacting roomfirst, and then the carbon source gas is introduced into the reactingroom with a carrier gas. The protecting gas comprises nitrogen gas,argon gas or other inert gas. The carbon source gas can be methane,ethane, acetylene and ethylene. The carrier gas is hydrogen gas.

In one embodiment, the protecting gas is argon gas. The carbon sourcegas is acetylene. The flow of the acetylene gas can be in a range fromabout 30 sccm to about 200 sccm. The flow of the hydrogen gas can be ina range from about 30 sccm to about 300 sccm. The pressure of thereacting room can be in a range from about 2 Torr sccm to about 760Torr. The flow rate of the carbon source gas and the carrier gas is in arange from about 0.1% to about 10%. The amorphous carbon depositionspeed is determined by the content of the carbon source gas in thereaction gas. The molar ratio of carbon source gas and carrier gaslower, the slower the deposition speed of the amorphous carbon. In oneembodiment, the flow rate of the carbon source gas and the carrier gasis in less than 5%. Thus, the deposition speed of the amorphous carboncan be slow down so as to obtain the carbon nanotubes having a cleansurface, and stronger van der Waals force therebetween.

In step (S143), in one embodiment, the heating temperature is in a rangefrom about 500° C. to about 740° C. The carbon source gas is introducedfor a time in a range from about 5 minutes to about 60 minutes to growthe carbon nanotube array 110 and the carbon nanotube cluster 108simultaneously. The carbon nanotube array 110 is grown on the secondcatalyst 106 and the carbon nanotube cluster 108 is grown on the firstcatalyst 104. The growth mechanism of the carbon nanotubes of the carbonnanotube array 110 and the carbon nanotube cluster 108 can be top growthmechanism or bottom growth mechanism.

The carbon nanotubes of the carbon nanotube array 110 are grown along adirection vertical to the growing surface 105 of the substrate 101. Thecarbon nanotube array 110 lifts the carbon nanotube layer 102 up awayfrom the growing surface 105. Thus, the carbon nanotube layer 102 isformed on a surface of the carbon nanotube array 110. The carbonnanotubes of the carbon nanotube array 110 are almost parallel with eachother. The height of the carbon nanotubes of the carbon nanotube array110 can be in a range from about 10 micrometers to about 900micrometers. The ends of the carbon nanotubes of the carbon nanotubearray 110 adjacent to the growing surface 105 are substantially form aflat surface. The portions of the carbon nanotubes of the carbonnanotube array 110 that are away from the growing surface 105 areentangled with each other.

The carbon nanotubes of the carbon nanotube cluster 108 are growndisorderly and intricately. The length of the carbon nanotubes of thecarbon nanotube cluster 108 can be in a range from about 10 micrometersto about 900 micrometers. The carbon nanotubes of the carbon nanotubecluster 108, the carbon nanotubes of the carbon nanotube layer 102, andthe portions of the carbon nanotubes of the carbon nanotube array 110,that are adjacent to the carbon nanotube layer 102, are entangled witheach other so that the carbon nanotube array 110, the carbon nanotubecluster 108, and the carbon nanotube layer 102 form a free standingintegrated structure. Thus, the carbon nanotube array 110 is firmlyfixed on the carbon nanotube layer 102.

Furthermore, an optional step (S15) of removing the carbon nanotubestructure 100 from the substrate 101 can be performed after step (S14).The carbon nanotube array 110, the carbon nanotube cluster 108 and thecarbon nanotube layer 102 can be removed from the substrate 101 togetherbecause they form a free standing integrated structure. In oneembodiment, the carbon nanotube structure can be peeled off from thesubstrate 101 via the carbon nanotube layer 102 easily.

Referring to FIG. 9, the carbon nanotube structure 100 of one embodimentfabricated by the method of FIG. 1 includes the carbon nanotube array110, the carbon nanotube cluster 108, and the carbon nanotube layer 102.

The carbon nanotube array 110 has a first surface 111 and a secondsurface 113 opposite to the first surface 111. The carbon nanotube array110 includes a plurality of first carbon nanotubes 115 that aresubstantially in parallel with each other. The plurality of first carbonnanotubes 115 extend from the first surface 111 to the second surface113. The carbon nanotube layer 102 is located on the first surface 11 ofthe carbon nanotube array 110. The carbon nanotube layer 102 includes aplurality of second carbon nanotubes 103. The plurality of second carbonnanotubes 103 are joined by van der Waals attractive force therebetweento form a free-standing structure. The carbon nanotube cluster 108 islocated on surface of the carbon nanotube layer 102. The carbon nanotubecluster 108 includes a plurality of third carbon nanotubes 107. Thethird carbon nanotubes 107 and the portions of the first carbonnanotubes 115 that are adjacent to the carbon nanotube layer 102 areentangled with each other and extend around the second carbon nanotubes103. Thus, the carbon nanotube array 110, the carbon nanotube cluster108, and the carbon nanotube layer 102 form a free standing integratedstructure. In one embodiment, the third carbon nanotubes 107 areentangled around the first carbon nanotubes 115 and the second carbonnanotubes 103 simultaneously. Each of the third carbon nanotubes 107 hasa first part entangled around the first carbon nanotubes 115 and asecond part entangled around the second carbon nanotubes 103.

In one embodiment, the carbon nanotube layer 102 includes two stackeddrawn carbon nanotube films as shown in FIG. 4. The aligned directionsof the carbon nanotubes of the two stacked drawn carbon nanotube filmsare substantially perpendicular with each other.

In one embodiment, the carbon nanotube structure 100 is observed byscanning electron microscope. Here, the side of the carbon nanotubestructure 100 adjacent to the carbon nanotube layer 102 is defined asbottom, and the side of the carbon nanotube structure 100 away from thecarbon nanotube layer 102 is defined as top. The SEM images of thecarbon nanotube structure 100 are shown in FIGS. 10-15. FIG. 10 is anSEM image of the carbon nanotube structure 100 in bend. FIG. 11 is anSEM image of side view of the carbon nanotube structure 100 of FIG. 10.FIG. 12 is an SEM image of top view of the carbon nanotube structure 100of FIG. 10. FIG. 13 is an SEM image of top side view of the carbonnanotube structure 100 of FIG. 10. FIG. 14 is an SEM image of bottomside view of the carbon nanotube structure 100 of FIG. 10. FIG. 15 is anSEM image of bottom view of the carbon nanotube structure 100 of FIG.10.

FIG. 10 shows that the carbon nanotube structure 100 is a free standingintegrated structure. The carbon nanotube structure 100 is flexible andcan be curved into arc shape. FIG. 11 shows that the carbon nanotubearray 110 is located on the carbon nanotube layer 102, and ends of thefirst carbon nanotubes 115 of the carbon nanotube array 110 are incontact with the carbon nanotube layer 102. FIG. 12 shows top ends ofthe first carbon nanotubes 115 of the carbon nanotube array 110. FIG. 13shows that the first carbon nanotubes 115 of the carbon nanotube array110 are substantially in parallel with each other. FIG. 13 shows thatthe extending direction of the first carbon nanotubes 115 issubstantially perpendicular with the extending direction of the secondcarbon nanotubes 103 of the carbon nanotube layer 102. FIG. 15 showsthat the third carbon nanotubes 107 of the carbon nanotube cluster 108are disordered and entangled with each other.

Referring to FIG. 16, a method for making a carbon nanotube structure200 of one embodiment includes the following steps:

step (S21), providing a substrate 201 having an growing surface 205 anddefining a plurality of holes 203;

step (S22), placing a carbon nanotube layer 202 on the growing surface205 of the substrate 201, wherein part of the growing surface 205 isexposed from the carbon nanotube layer 202;

step (S23), depositing catalyst so that a plurality of first catalyst204 is deposited on surface of the carbon nanotube layer 202 and aplurality of second catalyst 206 is deposited on the growing surface205;

step (S24), growing a carbon nanotube array 210 on the growing surface205 and growing a carbon nanotube cluster 208 on surface of the carbonnanotube layer 202; and

step (S25), removing the carbon nanotube structure 200 from thesubstrate 201.

The method for making the carbon nanotube structure 200 is similar tothe method for making the carbon nanotube structure 100 described aboveexcept that the substrate 201 defines a plurality of holes 203 and thecarbon nanotube array 210 is a patterned structure. Each of the holes203 can be a blind hole or through hole. The shape of the holes 203 canbe round, rectangle, triangle, or square. The plurality of holes 203 canbe arranged in an array. The step (S25) is optional.

When the holes 203 are through holes, the position of the substrate 201corresponding to the through holes 203 cannot have any catalyst. Thus,the carbon nanotube array 210 cannot grow from the position of thesubstrate 201 corresponding to the through holes 203. That is, thecarbon nanotube array 210 only grow from the position of the growingsurface 205 where has no hole to achieve a patterned carbon nanotubearray 210. When the holes 203 are blind holes, part of the secondcatalyst 206 will be deposited on the bottom surface of the blind holes203. Thus, the carbon nanotube array grown on the bottom surface of theblind holes 203 is lower than the carbon nanotube array 210 grown on thegrowing surface 205 and will not be in contact with and fixed on thecarbon nanotube layer 202. In step (S25) of removing the carbon nanotubestructure 200 from the substrate 201, the carbon nanotube array grown onthe bottom surface of the blind holes 203 will remain on the substrate201. That is, the patterned carbon nanotube array 210 can be obtained.The carbon nanotubes of the patterned carbon nanotube array 210 can havedifferent height.

Referring to FIG. 17, the carbon nanotube structure 200 of oneembodiment fabricated by the method of FIG. 16 includes the carbonnanotube array 210, the carbon nanotube cluster 208, and the carbonnanotube layer 202.

The carbon nanotube structure 200 is similar to the carbon nanotubestructure 100 described above except that the carbon nanotube array 210is a patterned structure. In one embodiment, the carbon nanotube layer202 includes two stacked drawn carbon nanotube films. The suspended partof the carbon nanotube layer 202 through the holes 203 is observed byscanning electron microscope. As shown in FIG. 18, a plurality ofdisordered and entangled carbon nanotubes are grown on a carbon nanotubestring of the drawn carbon nanotube film to form the carbon nanotubecluster 208, and no carbon nanotube array 210 is grown corresponding tothe through holes 203.

Referring to FIG. 19, a method for making a carbon nanotube structure300 of one embodiment includes the following steps:

step (S31), providing a carbon nanotube layer 302 including a pluralityof carbon nanotubes, and depositing a plurality of first catalyst 304 onthe carbon nanotube layer 302;

step (S32), providing a substrate 301 having an growing surface 305 anddepositing a plurality of second catalyst 306 on the growing surface305;

step (S33), placing the carbon nanotube layer 302 on the growing surface305;

step (S34), growing a carbon nanotube array 310 on the growing surface305 and growing a carbon nanotube cluster 308 on surface of the carbonnanotube layer 302; and

step (S35), removing the carbon nanotube structure 300 from thesubstrate 301.

The method for making the carbon nanotube structure 300 is similar tothe method for making the carbon nanotube structure 100 described aboveexcept that the step of depositing a plurality of first catalyst 304 onthe carbon nanotube layer 302 and the step of depositing a plurality ofsecond catalyst 306 on the growing surface 305 are performed separately.The step (S35) is optional.

The thickness of the carbon nanotube layer 302 can be above 500micrometers because the first catalyst 304 and the second catalyst 306are deposited separately. When the thickness of the carbon nanotubelayer 302 is above 500 micrometers, the first catalyst 304 is depositedon the surface of the carbon nanotube layer 302 adjacent to the growingsurface 305. In one embodiment, the thickness of the carbon nanotubelayer 302 is in a range from about 10 micrometers to about 100micrometers. Furthermore, the second catalyst 306 can be annealed in airat a temperature in a range from about 700° C. to about 900° C. for atime in a range from about 30 minutes to about 90 minutes, therebytransforming the catalyst layer into catalyst particles.

In one embodiment, the step of depositing a plurality of first catalyst304 on the carbon nanotube layer 302 can be omitted. The carbon nanotubelayer 302 without catalyst can be placed on the growing surface 305directly. Thus, no carbon nanotube cluster is grown on the carbonnanotube layer 302. The ends of the carbon nanotubes of the carbonnanotube array 310 away from the growing surface 305 are entangledaround the carbon nanotubes of the carbon nanotube layer 302.

Referring to FIG. 20, a method for making a carbon nanotube structure400 of one embodiment includes the following steps:

step (S41), providing a substrate 401 having an growing surface 405;

step (S42), growing a carbon nanotube array 410 on the growing surface405;

step (S43), providing a carbon nanotube layer 402 including a pluralityof carbon nanotubes, and depositing a plurality of first catalyst 404 onthe carbon nanotube layer 302;

step (S44), placing the carbon nanotube layer 402 on a surface of thecarbon nanotube array 410 away from the growing surface 405;

step (S45), and growing a carbon nanotube cluster 408 on surface of thecarbon nanotube layer 402; and

step (S46), removing the carbon nanotube structure 400 from thesubstrate 401.

The method for making the carbon nanotube structure 400 is similar tothe method for making the carbon nanotube structure 100 described aboveexcept that the step of growing the carbon nanotube array 410 and thestep of growing the carbon nanotube cluster 408 are performedseparately. The step (S46) is optional.

The result of the method enables easy peeling the carbon nanotube array410 from the substrate 401. The carbon nanotube cluster 408 can fix thecarbon nanotube array 410 on the carbon nanotube layer 402 firmly andenhance the electrical contact between the carbon nanotube array 410 andthe carbon nanotube layer 402. The carbon nanotube array 410 canwithstand larger electric field force and is suitable for field emissiondevice.

Referring to FIG. 21, a field emission device 10 using the carbonnanotube structure 100 of one embodiment includes the carbon nanotubestructure 100, two electrodes 112 electrically connected with the carbonnanotube structure 100, and an anode electrode 114 spaced from thecarbon nanotube structure 100. The carbon nanotube structure 100includes the carbon nanotube array 110, the carbon nanotube cluster 108,and the carbon nanotube layer 102. The carbon nanotube structure 100 canbe replaced by the carbon nanotube structure 200, the carbon nanotubestructure 300, or the carbon nanotube structure 400.

The two electrodes 112 are spaced from each other. The carbon nanotubelayer 102 is located on and in contact with surfaces of the twoelectrodes 112. Part of the carbon nanotube layer 102 between the twoelectrodes 112 is suspended. The carbon nanotubes of the carbon nanotubearray 110 extend along a direction that is perpendicular to the anodeelectrode 114. Furthermore, an optional gate electrode (not shown) canbe located between the carbon nanotube structure 100 and the anodeelectrode 114, so as to control the emission. The shapes of the twoelectrodes 112 are not limited, and the two electrodes 112 can be madeof a conductive material, such as metal. The anode electrode 114 is aconductive layer, such as a metal layer, an indium tin oxide (ITO)layer, or a carbon nanotube layer. The thickness of the anode electrode114 can be selected according to need. In one embodiment, the twoelectrodes 112 are two parallel nickel rods, the carbon nanotubestructure 100 is suspended between the two nickel rods as shown in FIG.22.

In use, the field emission device 10 is located in a chamber (not shown)in vacuum with a pressure lower that 10⁻⁵ Pa or filled with inert gas.In one embodiment, the carbon nanotube structure 100 is grounded, and apositive voltage is applied to the anode electrode 114. Thus, apotential difference is obtained between the carbon nanotube structure100 and the anode electrode 114. The carbon nanotubes of the carbonnanotube array 110 will emit electrons under the electric field force.Because the carbon nanotube cluster 108 fix the carbon nanotube array110 on the carbon nanotube layer 102 firmly, the carbon nanotubes of thecarbon nanotube array 110 can withstand larger electric field force andwill not be pulled out. Furthermore, a voltage is applied between thetwo electrodes 112 so that a current flow through the carbon nanotubestructure 100 will heat the carbon nanotubes of the carbon nanotubearray 110 during the process of field electron emission. Thus, the gasabsorbed in the carbon nanotube array 110 will be removed by heating andthe carbon nanotube array 110 can emit electrons more stably. Becausepart of the carbon nanotube structure 100 is suspended between the twoelectrodes 112 and the heat capacity per unit area of the carbonnanotube structure 100 is low, the field emission device 10 has a highheating response speed.

In one embodiment, the carbon nanotube structure 100 is heated by pulsedheating signal. FIGS. 23-26 show the testing results of the fieldemission device 10 of FIG. 22. The field emission device 10 is tested atroom temperature and 1240K respectively. The testing sample carbonnanotube structure 100 has a length of about 8 millimeters, a width ofabout 2 millimeters, a length of about 100 micrometers, and a resistanceof about 400 ohms. As shown in FIG. 23, the field emission device 10 canform a field emission current under a voltage of about 400V at roomtemperature or at 1240K. The field emission current at 1240K is a smoothcurve indicates that the field emission current at 1240K is more stable.As shown in FIG. 24, the temperature of the testing sample can ramp up1193K in 84 milliseconds when a pulsed heating signal is applied andcool down to room temperature in 42 milliseconds when the pulsed heatingsignal is stop. The pulsed heating voltage is about 30.8V and the pulsedheating temperature is about 1193K. The pulsed heating duty ratio of 5%,10%, 30%, 50%, 70%, and 90% are used respectively. The pulsed heatingduty ratio is a ratio t/T between a time width “t” of the pulsed heatingsignal and a time cycle “T” of the pulsed heating signal. As shown inFIGS. 25-26, the pulsed heating duty ratio of 5% is enough to causeobvious desorption. The field emission device 10 can achieve a stableadsorbent-free field emission with a heating power of about 0.3 W atpulsed heating duty ratio of 10%.

Referring to FIG. 27, a field emission device 20 using the carbonnanotube structure 200 of one embodiment includes an insulativesubstrate 216, the carbon nanotube structure 200 located on theinsulative substrate 216, two electrodes 212 electrically connected withthe carbon nanotube structure 200, and an anode electrode 214 spacedfrom the carbon nanotube structure 200. The carbon nanotube structure200 includes the patterned carbon nanotube array 210, the carbonnanotube cluster 208, and the carbon nanotube layer 202. The carbonnanotube structure 200 can be replaced by the carbon nanotube structure100, the carbon nanotube structure 300, or the carbon nanotube structure400.

The field emission device 20 is similar to the field emission device 10described above except that the carbon nanotube structure 200 is locatedon the insulative substrate 216. In one embodiment, the carbon nanotubelayer 202 is in contact with the insulative substrate 216, the twoelectrodes 212 are located on the carbon nanotube structure 200, andparts of the carbon nanotube structure 200 are located between the twoelectrodes 212 and the insulative substrate 216 so that the carbonnanotube structure 200 is fixed on the insulative substrate 216 firmly.

Referring to FIG. 28, a field emission device 30 using the carbonnanotube structure 100 of one embodiment includes a column shapedsubstrate 316, the carbon nanotube structure 100 located on the columnshaped substrate 316, and an anode electrode 314 spaced from the carbonnanotube structure 100.

The field emission device 30 is similar to the field emission device 20described above except that the substrate 316 is column shaped. Thecarbon nanotube structure 100 is located around the outer surface of thecolumn shaped substrate 316. The anode electrode 314 is a hollow tubearound the carbon nanotube structure 100. The substrate 316 can be acylinder, a triangular prism, or a quadrangular prism. The sectionalshape of the anode electrode 314 can be triangular, circular, or square.In one embodiment, the substrate 316 is a ceramic cylinder; the carbonnanotube structure 100 is located around and in contact with the outersurface of the substrate 316; and the anode electrode 314 is quartz tubewith an ITO layer on the inner or outer surface. The carbon nanotubes ofthe carbon nanotube array 110 extend from the substrate 316 to the anodeelectrode 314, along an extending direction that is perpendicular withthe surface of the anode electrode 314. Because the carbon nanotubestructure 100 is flexible, the carbon nanotube structure 100 can belocated on the surface in any shape. Furthermore, the field emissiondevice 30 can include two electrodes located on two ends of thesubstrate 316 and electrically connected with the carbon nanotubestructure 100.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A carbon nanotube structure being a free-standingstructure, comprising: a carbon nanotube array, wherein the carbonnanotube array comprises a plurality of first carbon nanotubes that aresubstantially parallel to each other; a carbon nanotube layer located onone side of the carbon nanotube array, wherein the carbon nanotube layercomprises a plurality of second carbon nanotubes in contact with a firstportion of each of the plurality of first carbon nanotubes; and aplurality of third carbon nanotubes entangled and located between thecarbon nanotube layer and the carbon nanotube array, wherein the carbonnanotube array is fixed on the carbon nanotube layer by the plurality ofthird carbon nanotubes so that the carbon nanotube structure isfree-standing.
 2. The carbon nanotube structure of claim 1, wherein thecarbon nanotube layer comprises a drawn carbon nanotube film, and theplurality of second carbon nanotubes are arranged substantially alongthe same direction.
 3. The carbon nanotube structure of claim 2, whereinthe plurality of second carbon nanotubes are successively joinedend-to-end by van der Waals attractive force therebetween.
 4. The carbonnanotube structure of claim 1, wherein the carbon nanotube layercomprises a flocculated carbon nanotube film, the plurality of secondcarbon nanotubes are entangled with each other.
 5. The carbon nanotubestructure of claim 1, wherein the carbon nanotube layer comprises apressed carbon nanotube film, and an angle between a primary alignmentdirection of the plurality of second carbon nanotubes and a surface ofthe pressed carbon nanotube film is in a range of approximately 0degrees to approximately 15 degrees.
 6. The carbon nanotube structure ofclaim 1, wherein the carbon nanotube layer comprises a plurality ofuntwisted carbon nanotube wires, the plurality of second carbonnanotubes are substantially oriented along a length direction of each ofthe plurality of untwisted carbon nanotube wires.
 7. The carbon nanotubestructure of claim 1, wherein the carbon nanotube layer comprises aplurality of twisted carbon nanotube wires, the plurality of secondcarbon nanotubes are substantially helically oriented around an axialdirection of each of the plurality of twisted carbon nanotube wires. 8.The carbon nanotube structure of claim 1, wherein the plurality ofsecond carbon nanotubes extend substantially along a direction that isparallel to a surface of the carbon nanotube layer and perpendicular tothe plurality of first carbon nanotubes.
 9. The carbon nanotubestructure of claim 1, wherein the plurality of third carbon nanotubesare entangled around both the plurality of first carbon nanotubes andthe plurality of second carbon nanotubes.
 10. The carbon nanotubestructure of claim 9, wherein each of the plurality of third carbonnanotubes has a first part entangled around the plurality of firstcarbon nanotubes and a second part entangled around the plurality ofsecond carbon nanotubes.
 11. The carbon nanotube structure of claim 1,wherein the first portions of the plurality of first carbon nanotubesare entangled around the plurality of second carbon nanotubes.
 12. Thecarbon nanotube structure of claim 1, wherein the carbon nanotube arrayis a patterned structure defining a plurality of through holes, and thecarbon nanotube layer covers the plurality of through holes.
 13. Acarbon nanotube structure being a free-standing structure, comprising: acarbon nanotube layer, wherein the carbon nanotube layer comprises aplurality of first carbon nanotubes extending substantially along adirection parallel to a surface of the carbon nanotube layer; a carbonnanotube array located on and in contact with the surface of the carbonnanotube layer, wherein the carbon nanotube array comprises a pluralityof second carbon nanotubes that are parallel to each other andperpendicular to the surface of the carbon nanotube layer; and a carbonnanotube cluster located between the carbon nanotube layer and thecarbon nanotube array, wherein the carbon nanotube cluster comprises aplurality of third carbon nanotubes that are entangled around both theplurality of first carbon nanotubes and the plurality of second carbonnanotubes so that the entire carbon nanotube structure is free-standing.14. The carbon nanotube structure of claim 13, wherein the carbonnanotube array is a patterned structure defining a plurality of throughholes, and the carbon nanotube layer covers the plurality of throughholes.
 15. The carbon nanotube structure of claim 14, wherein a firstportion of each of the plurality of second carbon nanotubes is entangledaround one of the plurality of first carbon nanotubes.
 16. A carbonnanotube structure, comprising: a substrate comprising a surface; acarbon nanotube array located on the substrate, wherein the carbonnanotube array has comprises a plurality of first carbon nanotubes thatare substantially parallel to each other and perpendicular to thesurface of the substrate; a carbon nanotube layer located on and incontact with the carbon nanotube array so that the carbon nanotube arrayis sandwiched between the substrate and the carbon nanotube layer,wherein the carbon nanotube layer comprises a plurality of second carbonnanotubes in contact with the plurality of first carbon nanotubes; and aplurality of third carbon nanotubes entangled and located between thecarbon nanotube layer and the carbon nanotube array, wherein the carbonnanotube array is fixed on the carbon nanotube layer by the plurality ofthird carbon nanotubes so that the entire carbon nanotube structure isfree-standing.
 17. The carbon nanotube structure of claim 16, whereinthe substrate comprises a plurality of first through holes, and thecarbon nanotube array is a patterned structure and comprises a pluralityof second through holes aligned with the plurality of first throughholes.
 18. The carbon nanotube structure of claim 17, wherein theplurality of first through holes is free of carbon nanotubes.
 19. Thecarbon nanotube structure of claim 17, wherein the carbon nanotube arrayis not in the plurality of first through holes.
 20. The carbon nanotubestructure of claim 16, wherein the substrate is selected from the groupconsisting of silicon substrate, silicon dioxide substrate, quartzsubstrate, and sapphire substrate.